(758d) The Use of Isotopic Labels In Understanding CO/H2 Reactions

The reaction
between carbon monoxide and hydrogen can be catalysed to give a variety of
products including methane, methanol, acetic acid and waxes.  The mechanisms
involved are rarely simple and using ¹³C, ¹⁸O, and D₂
as tracers has helped to determine reaction paths.  Often a combination of
isotopes is needed to fully specify the mechanism. In this talk we will look at
complex systems where there is the potential for multiple products and where
understanding the mechanism is critical to catalyst optimisation.

The synthesis of methanol from carbon
monoxide-hydrogen and carbon monoxide carbon dioxide-hydrogen over a
copper/zinc oxide/alumina catalyst was studied using transient isotope tracing
techniques.  The non-steady-state period immediately after start-up has been
studied and the product distribution and time delay could be explained with
reference to the amount of residual oxygen on the catalyst after reduction. 
Using [^l8O]carbon monoxide and [¹⁸O]carbon dioxide it was
shown the label is not detected in the methanol product for 0.3 h.  From the
steady-state activities and a residence time of 0.3 h the size of the surface
reservoir of methanol precursor was calculated for both carbon
monoxide-hydrogen and carbon monoxide-carbon dioxide/hydrogen feedstreams. 
This figure was in good agreement with the amounts of methanol removed from the
catalyst when the feedstream was switched from carbon monoxide-hydrogen or
carbon monoxide-carbon dioxide-hydrogen to hydrogen alone.

The production of
oxygenate species over rhodium catalysts can under the correct conditions
produce a range of oxygenate species including methanol, ethanol, acetaldehyde
and acetic acid.  Using [¹³C]CO, [¹⁸O]CO, and D₂
the mechanism of formation of each species was determined.  When labeled carbon monoxide was
introduced, neither methane nor ethanol nor methanol showed any incorporation;
however, the labels were rapidly incorporated into the aldehydic function of
ethanal.  Labeled water, produced from the hydrogenation of [¹⁸O]CO,
took more than 0.3 h to desorb.  The results suggest that (i) ethanol and
ethanal are produced independently with no common intermediate, (ii) the
formation of alcohols is slow (taking over 0.5 h), and
(iii) carbon monoxide is not hydrogenated directly to methane but goes through
the hydrocarbonaceous residue present on the surface.  The carbonaceous residue
was found to play a central role in the mechanism, supplying in effect both
hydrogen and CH2 units to other reactive surface species.

The details of
these and other studies will be reported showing the unrivalled access to
mechanistic information that isotopes can give in the field of catalysis.